I must admit that the first time I saw your piece, I was a little uncomfortable because of the elf on the foreground, who's making a really "weird" sign, if you know what I'm refering to. So I'm sure you didn't even think about that, but I would've slightly changed the sign to not be so alike.
Technically, I think it's a really great work for a first time with a graphic tablet ! Those slightly desaturated christmas colors are really cool. I like that you didn't neglect the background and the town and forest and mountains really add life and consistence to your piece.
Regarding your intention to depict the elves "combating the growing rise of consumerism and online shopping that is displacing them." I would've maybe made it more obvious. The main focus of the viewer's eyes go to the main elf and then to the crowd. At first I didn't get that the intention of the poster was to fight against online shopping, I had to read your description for that.
Finally, the right bottom part is really empty and feels like the poster was designed too big for the message. I would've work on this part to be more focused on the online shopping issue. Maybe it was a lack of time on you part ?
Good luck for the competition, and well done !